1. Field of the Invention
This invention relates to the construction and operation of rare gas halide excimer lasers capable of generating optical pulse trains characterized by a high duty factor (ratio of pulse duration to pulse repetition period). Application of prior art in rare gas halide laser technology has resulted in lasers with pulse durations in the 10 to 1000 nanosecond range and pulse repetition rates of less than 10 kHz with corresponding duty factors of less than 1 percent. We here disclose a new laser geometry that allows operation of rare gas halide lasers with greatly extended pulse durations and pulse repetition rates so that the laser output more closely approximates continuous emission.
The invention comprises features of construction, combinations of elements, and arrangements of parts forming a rare gas halide laser structure capable of high duty factor operation.
2. Object of the Invention
An object of the invention is to provide a simple technique for generation of continuous or quasi-continuous laser radiation in the ultraviolet portion of the spectrum.
A second object of the invention is to provide a structure for excitation of rare gas halide laser gases that utilizes atomic and molecular processes occuring at the walls of the structure to extend the laser pulse duration and pulse repetition rate.
A third object of the invention is to provide a laser excitation structure that allows very high pulse repetition rates without need for rapid flow of the laser gas through the discharge region.
A fourth object of the invention is to provide a laser excitation structure that allows operation of a rare gas halide laser with laser pulse duration exceeding 100 nanoseconds.
A fifth object of the invention is to provide a rare gas halide laser excitation structure capable of withstanding exposure to halogen gases thereby providing of extended operating and storage lifetime.
A sixth object of the invention is to provide a rare gas halide laser excitation structure capable of removing heat from the discharge region at a rate adequate to allow high duty factor operation without gas flow.
3. Description of Prior Art
Newman (U.S. Pat. No. 4,381,564) has disclosed rare gas halide waveguide lasers with pulsed dc excitation capable of ultraviolet laser emission with pulse durations of less than 30 nanoseconds and pulse repetition rates potentially extending to 10 kHz. The corresponding maximum duty factor for this laser is thus limited to 0.03 percent. Since excitation of this device is intrinsically of short duration, higher duty factors cannot be achieved by extension of pulse duration. Pulse repetition rates in the Newman device are limited by gas heating and halogen donor recombination processes discussed in later paragraphs.
Christensen and Waynant (Appl. Phys. Lett, 41, 794 (1982) have suggested the utility of electrodeless radio frequency excitation for long pulse rare gas halide excimer lasers and described a xenon fluoride laser with a relatively inert ceramic discharge tube that operated at an undisclosed repetition rate with a pulse duration limited to approximately 300 nanoseconds by halogen donor dissociation. However, these authors did not consider design features necessary for high repetition rate and high duty factor operation.
Christensen, Waynant, and Feldman (Appl. Phys. Lett. 46, 321 (1985) demonstrated an electrodeless discharge XeCl laser with a pulse duration of 320 nanoseconds and suggested the feasibility of operation at repetition rates of tens of kilohertz (resulting in duty factors of less than 1%). Christensen discloses a similar device in U.S. Pat. No. 4,631,732.
4. Technical Background
All devices associated with the prior art are intrinsically limited in either pulse duration or pulse repetition rate to duty factors of less than 1%. This limit is imposed by duration of the pulsed excitation, dissociation of the halogen donor molecules in the gas discharge, and removal of waste heat from the discharge region. Use of electrodeless rf or microwave discharges has been shown to allow homogeneous, long-term excitation of a high pressure gas, however, problems associated with halogen donor recombination and heat removal previously have not been addressed in a manner that allows cw or quasi-cw operation.
Halogen donor dissociation is a process that is essential to the operation of a rare gas halide laser. The excited rare gas halide molecule which produces optical gain is normally formed by reaction of a halide compound (hydrogen chloride, bromine, fluorine, nitrogen trifluoride, or other suitable halogen bearing molecule) which serves as a halogen donor with an appropriate excited rare gas atom. After emitting a laser photon the rare gas halide molecule rapidly dissociates to yield a halogen atom and a rare gas atom. However, recombination of the halogen atom with the corresponding molecular fragment to reform the halogen donor molecule is a relatively slow three-body process in the bulk gas and typically requires approximately 100 microseconds. In the electric discharge used for laser excitation the halogen donor molecule is normally dissociated much faster than it can recombine in the bulk gas. When the halogen donor concentration falls below a certain level laser operation stops, and excitation of the gas must be halted for a period adequate to allow recombination. Laser pulse duration is typically limited to a few hundred nanoseconds and laser pulse repetition rate limited to a few tens of kilohertz. Consequently lasers of the prior art that do not utilize rapid gas flow and rely on halogen donor recombination in the bulk gas are intrinsically limited to duty factors of approximately 1%.
Waste heat generation in the discharge region also limits the duty factor achievable with rare gas halide lasers associated with prior art. At excitation rates sufficient to produce useful optical gains waste heat generated in a rare gas halide laser discharge would heat the laser gas at a rate exceeding 1 million degrees Centigrade per second if it were thermally insulated from its surroundings. Although some rare gas halide laser systems are operable at temperatures of several hundred degrees Centigrade, higher temperatures are deleterious to laser action and often introduce refractive index variations that distort the laser beam. Efficient cooling is thus necessary for high duty factor operation.